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Amine-Assisted Route To Fabricate LiNbO3 Particles with a Tunable Shape Meinan Liu and Dongfeng Xue* State Key Laboratory of Fine Chemicals, Department of Materials Science and Chemical Engineering, School of Chemical Engineering, Dalian UniVersity of Technology, 158 Zhongshan Road, Dalian 116012, P. R. China ReceiVed: January 28, 2008; In Final Form: February 19, 2008
An ethylenediamine-assisted route has been designed for one-step synthesis of lithium niobate particles with a novel rodlike structure in an aqueous solution system. The morphological evolution for these lithium niobate rods was monitored via SEM: The raw materials form large lozenges first. These lozenges are a metastable intermediate of this reaction, and they subsequently crack into small rods after sufficiently long time. These small rods recrystallize and finally grow into individual lithium niobate rods. Interestingly, shape-controlled fabrication of lithium niobate powders was achieved through using different amine ligands. For instance, the ethylenediamine or ethanolamine ligand can induce the formation of rods, while n-butylamine prefers to construct hollow spheres. These as-obtained lithium niobate rods and hollow spheres may exhibit enhanced performance in an optical application field due to their distinctive structures. This effective ligand-tunedmorphology route can provide a new strategy to facilely achieve the shape-controlled synthesis of other niobates.
Introduction In recent years, a challenge in materials engineering is the controlled fabrication of inorganic materials with purposely designed shapes to provide an increasingly precise control over the structures and properties.1,2 One-dimensional (1D) nanostructures such as rods,3 wires,4 tubes,5 and belts6 have attracted immense interest because of their distinctive geometries, novel physical and chemical properties, and potential applications in nanodevices. The general approach for fabrication of most 1D materials has involved the use of various templates or capping reagents.7 Despite great advances in these template methods, the synthesis of multicomponent substances like lithium niobate (LiNbO3, LN) in aqueous solution without a template seems more interesting in terms of operation cost, environment pollution, and the potential for large-scale production. Therefore, the search for a new synthetic strategy to achieve 1D materials in an aqueous solution system is essential and significant. As an important functional material with unique electrooptical, piezoelectric, and nonlinear optical properties combined with good mechanical stability, LN has received considerable attention.8,9 Up to now, various preparation methods for LN powders have been reported, such as combustion reaction,10 sol-gel,11 metal alkoxides,12 Pechini method,13 nonhydrolytic solution reaction,14 and hydrothermal15 and solvothermal16 processes as well as the peroxide route.17 Although these methods are available to synthesize LN powders, the as-obtained LN products with novel microstructures have been rarely reported due to its complexity of crystal nucleation and growth process. Recently, a solvent coordination molecular template method has been demonstrated to be an efficient pathway to fabricate materials with various shapes, since special structures and fascinating self-assembling functions of these organic ligands allow them to serve as templates for the design and preparation of complicated structures.18 For instance, in ethylenediamine or 1,6-diaminohexane bidentate solvents, Cd-based * To whom correspondence should be addressed. E-mail: dfxue@ chem.dlut.edu.cn.
chalcogenides nanorods have been obtained.19 Therefore, we attempt to design a ligand-assisted aqueous solution system for deliberately fabricating LN with controllable structures. In this work, we develop a facile ethylenediamine-assisted method for preparing LN microscale rods in aqueous solution without a template. Furthermore, it has been found that the amine ligand5,20 has a very significant effect on nucleation and growth of LN products. Consequently, shape-controlled synthesis of LN products can be achieved through deliberately selecting different amine ligands. For instance, ethylenediamine and ethanolamine can be in favor of forming a rodlike structure, while n-butylamine prefers to construct a hollow sphere structure. This ligand-tuned-morphology strategy will promote a better understanding of LN crystal growth, and moreover, the as-synthesized LN rods and hollow spheres may lead to many new potential applications. Experimental Section The starting materials, Nb2O5 (99.99% purity), LiOH (AP), and amine ligands (i.e., ethylenediamine, n-butylamine, triethylamine, and ethanolamine), were used as supplied. In a typical procedure, freshly prepared niobic acid (the detailed synthetic processes of niobic acid from Nb2O5 have been described in our previous work16) with double molar ratio of LiOH was added to the mixture at various volume ratios of amine/water (detailed experimental conditions are shown in Table S1). Subsequently, the white suspension was placed into a Teflon-lined stainless steel autoclave of 50 mL capacity up to 70% of the total volume. The autoclave was maintained at 220 °C for 1-4 days without shaking or stirring during the heating period and then naturally cooled to room temperature. A white precipitate was collected and then washed with distilled water and ethanol to remove the residue of ethylenediamine. The final product was dried at 60 °C for 5 h in air. The morphology and crystal structure of the obtained powders were investigated using scanning electron microscopy (SEM, JSM-5600LV, JEOL) and X-ray diffraction (XRD, D/Max 2400, Rigaku) techniques, respectively. Thermogravimetric analysis
10.1021/jp800803s CCC: $40.75 © 2008 American Chemical Society Published on Web 04/02/2008
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Figure 1. XRD pattern of LN obtained in 5 mL of ethylenediamineassisted reaction system at 220 °C for 4 d. The standard diffraction pattern of LN (JCPDS card No. 85-2456) is shown as a reference.
and differential scanning calorimetry (TGA/DSC, SDT Q600, TA) were employed to analyze the thermal behaviors of the as-synthesized intermediate in N2 atmosphere at a heating rate of 10 °C/min. Additionally, infrared spectra (IR) of the intermediate were measured by the KBr pellet method (NEXUS, Nicolet) in the range 400-4000 cm-1. Energy dispersive X-ray spectroscopy (EDS) was also used to investigate the possible composition of the as-prepared intermediates. UV/vis spectra were obtained using a UV/vis-NIR spectrophotometer (JASCO, V-570). Results and Discussion The XRD pattern of the obtained LN products from the ethylenediamine-assisted reaction system shown in Figure 1 exhibits diffraction peaks corresponding to a hexagonal structure. The values of lattice constants, a ) b ) 5.136 and c ) 13.84 Å, are well-consistent with that of a ) b ) 5.148 and c ) 13.85 Å from JCPDS 85-2456. No diffraction peaks arising from impurities such as Nb2O5 were detected, indicating the high purity of the as-synthesized products. Figure 2 shows typical SEM images of the obtained LN powders with uniform rodlike morphology. The high magnification image clearly displays these rods with the diameter 0.8-1 µm and the length 2.5-3 µm. In this study, the hydrothermal reaction between niobic acid and lithium hydroxide in various volume ratios of ethylenediamine/water solution for a proper reaction time yields very interesting results. The reaction time and ethylenediamine volume play crucial roles in controlling the nucleation and growth of crystallites. XRD patterns of the solids sampled after various periods of the reaction at 220 °C are presented in Figure 3, which indicates pronounced changes in crystal structure during the course of the reaction. The patterns of the sample obtained in 1.5 d are distinctly different from that of Nb2O5 or LN, indicating that a new crystal phase forms at the expense of niobic acid in a short period. The new phase of the large particles observed in SEM images shows a lozenge shape with the edge length of about 10-15 µm (Figure 4a). An overall morphology reveals that the obtained lozenges are uniform and monodispersed, as shown in Figure S1. The lozenges are of particular interest because of their crystal and morphological features. To further understand these lozenges, their thermal behavior and IR spectra were investigated. As shown in Figure S2, the weight loss of this intermediate product is approximately 10.7% in the
Figure 2. Typical SEM images of LN rods: (a) general view of LN rods showing the obtained powders with uniform morphology; (b) magnified SEM image of LN rods obtained in 5 mL of ethylenediamineassisted reaction system at 220 °C for 4 d.
Figure 3. XRD patterns of LN and its precursor obtained with different reaction times. The diffraction peak marked with an asterisk is attributed to LN.
temperature range from 50 to 800 °C. On the other hand, one endothermic peak centered at about 150 °C and one exothermic peak centered at about 518 °C can be observed in the DSC curve. The broad endothermic peak can be attributed to the loss of water and the surface ethylenediamine molecule, while the sharp exothermic peak centered at 518 °C can be attributed to the decomposition of intermediates into LN. IR spectra of Figure S3 exhibit stronger absorption at 3450 and 1635 cm-1 in lozenge samples compared with that of pure LN samples, further confirming the presence of the hydroxyl group in these lozenges, which is well-consistent with the analysis of TG results. Obviously, in the IR fingerprinting region, the absorption bands
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Figure 4. SEM images of the structure evolution of LN with increasing reaction times: (a) 1.5 d; (b) 2 d; (c) 2 d; (d) 2.5 d; (e) 3 d; (f) 4 d.
are obviously different between the intermediate and pure LN samples. There is only one broad Nb-O absorption band at approximately 677 cm-1 in the LN samples. However, three narrow bands exist in lozenges, centered at 800, 680, and 500 cm-1, which can well demonstrate the different Nb-O chemical environment in the as-prepared intermediates. In Figure S4, EDS analysis clearly reveals the presence of Nb and O with an atomic ratio close to 1:4 (H and Li cannot be detected by EDS measurements). Combined with the calculation of 10.7% weight loss in TG results, it can be concluded that the as-obtained intermediate compounds consist of 4.2% Li, 56.0% Nb, 38.6% O, and 1.2% H (by weight). Further study indicates that these lozenge samples appear as a metastable intermediate and the crystallinity degrades readily in the subsequent reaction process. For instance, as shown in Figure 3, some diffraction peaks of LN appear after 2 d of reaction. In addition, SEM results also demonstrate this process. With a prolonging of the reaction time, it can be clearly observed that the surface of the lozenges begins to dissolve (see Figure 4b) and some lozenges crack into numerous small rods (Figure 4c). Figure 4 parts d (2.5 d) and e (3 d) reveal the presence of a large number of rods, which indicates that the purity of rods in the product is increased by prolonging the reaction time. XRD results definitely confirm that the products obtained after 3 d of reaction are pure LN. Nonetheless, the diffraction peaks are broad and show low intensity, indicating poor crystallinity in these rodlike solids (Figure 4e). The samples prepared in 4 d are well-crystallized rods, as shown in Figure 4f, and the high intensity of diffraction peaks (Figure 3) indicates a high crystallinity of these perfect rods. On the other hand, the phase structure and morphology of products varied greatly as ethylenediamine volume changes from 1 to 25 mL while keeping the other conditions constant. XRD patterns of powders fabricated with different ethylenediamine volumes are shown in Figure 5. When 1 mL of ethylenediamine was introduced into this aqueous solution system, the samples obtained were found to be intermediates of LN since its diffraction peaks are definitely identical with those of lozenges, indicating that scarcity of ethylenediamine limits the transformation from intermediate into LN product. With increasing ethylenediamine to 3 mL, LN powders can be obtained. Further
Figure 5. XRD patterns of LN and its intermediate obtained with the assistance of different volumes of ethylenediamine.
increasing ethylenediamine volume, the crystal phase of the products did not change, which could be confirmed by XRD results (Figure 5). Figure 6a-f shows SEM images of products obtained with different ethylenediamine volumes. There is a large difference between the intermediate and LN product. Only irregular particles were observed in the intermediate samples prepared with ethylenediamine volume ) 1 mL, as shown in Figure 6a. When ethylenediamine volume reached up to 3-10 mL, a large number of rods in LN products were found but the diameter of the as-obtained rods increased with increasing ethylenediamine volume (see Figure 6b-d). Further increasing ethylenediamine volume to 15-25 mL, numerous quasi-spheres were observed. Our results indicate that ethylenediamine volume plays an important role in controlling nucleation and growth of LN rods. An appropriate ethylenediamine volume is critical for the construction of LN with a rodlike structure. A possible growth process in ethylenediamine-assisted system is schematically illustrated in Figure 7. On the basis of the observation from XRD patterns and SEM images, it is reasonable to conclude that, in our approach, Nb groups coordinate with ethylenediamine in LiOH solution under a hydrothermal system to form an LN intermediate with lozenge structure that
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Figure 6. SEM images of LN obtained in different volumes of ethylenediamine: (a) 1 mL; (b) 3 mL; (c) 5 mL; (d) 10 mL; (e) 15 mL; (f) 25 mL. All scale bars ) 1 µm.
Figure 7. Possible mechanism for the formation of uniform LN rods.
then cracks into small rods at sufficiently high temperatures and long times. Since this process occurred in ethylenediamine solution, a dynamical equilibrium between the intermediate and LN phases should be expected, and this offered the as-formed small rods the chance to recrystallize and finally grow into individual rods. In the case of a great deal of ethylenediamine in solution, the quasi-spheral structure is obtained as shown in Figure 6, which may be caused by the overaggregation of these small rods. Therefore, it can be concluded that ethylenediamine plays dual roles in the designed system for fabrication of LN rods. First, ethylenediamine coordinates with Nb group to form a stable but active niobium-amine complex and, thus, lowers the activation-energy barriers of LN nucleation. In addition, ethylenediamine acts as a structure-directing molecule that is incorporated into the inorganic framework first19 and then escapes from it to form LN with a rodlike structure. The versatile organic amine is a superior class of coordination reagent due to its strong coordination ability with transition metals, especially with Nb.5,20 Herein, other organic amines, such as n-butylamine, triethylamine, and ethanolamine, have also been applied for the crystallization of LN. Figure 8a shows
that LN particles crystallized in an n-butylamine-assisted reaction system are hollow spheres with diameters of about 1.5-2 µm. From the triethylamine-assisted reaction system, the formed particles are monodisperse spheres with diameters of about 400 nm (Figure 8b). Unfortunately, these spheres are not LN, as revealed by XRD results (see Figure 8e). In a mixture of ethanolamine and water, LN rods are formed, which are similar in shape to those obtained in an ethylenediamine/water mixture (Figure 8c). These results demonstrate that amine ligands can provide an effective way for the controllable preparation of LN with hollow spheres and rodlike structures. The constructed spheral and rodlike structure may be dependent on the special properties and geometries of different amine ligands. As shown in Figure S5, n-butylamine is a monodentate ligand; while the geometry of triethylamine is complex and displays strong steric hindrance, ethylenediamine and ethanolamine are bidentate ligands and, moreover, their structures are almost identical. As experimental results demonstrated, LN could not be fabricated from a triethylamineassisted reaction system, because its strong steric hindrance hampers the coordination with Nb group as well as further
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Figure 8. SEM images and XRD patterns of the obtained powders with n-butylamine (a and d), triethylamine (b and e), and ethanolamine (c and f) instead of ethylenediamine as organic ligand.
and hollow spheres are expected to exhibit superior optical properties due to their unique structures. Conclusion
Figure 9. UV/vis diffuse reflectance spectra of the obtained LN rods and hollow spheres.
chemical reaction for LN. In comparison with n-butylamine, ethylenediamine and ethanolamine can be favored for constructing a rodlike structure, mainly because they contain more than one chelating atom in each molecule, which can form LN crystallites with a 1D structure. UV/vis spectra of the as-synthesized LN with rodlike and hollow sphere morphology are illustrated in Figure 9. From our UV spectra, it is clearly observed that the absorption edge of these hollow spheres shows a blue shift by 15 nm, compared with that of rods. It has been widely accepted that the size increase is accompanied by changes in the optical properties. Therefore, in our case, the blue shift can be attributed to a weak quantum confinement effect. As UV results shown, these rods
In this work, we have demonstrated a simple yet versatile procedure for one-step growing LN rods with the assistance of bidentate ethylenediamine or ethanolamine ligand in aqueous solution. The as-synthesized LN rods had nearly uniform diameters and a smooth surface with an average diameter of ∼0.8 µm and length up to 3 µm. Furthermore, deliberate control of LN with 1D and 3D structures has been achieved by using different amine ligands. Experimental results well demonstrate that bidentate ethylenediamine and ethanolamine form rodlike structures, while monodentate n-butylamine fabricates hollow spheres. This suggests that the as-obtained LN rods and hollow spheres may exhibit enhanced applications in pyroelectric sensors, acoustic-optic modulators, and optical devices. Additionally, this effective ligand-tuned-morphology route can provide a new strategy to facilely achieve the shape-controlled synthesis of other niobates. Acknowledgment. The financial support from the Program for New Century Excellent Talents in the University (NCET05-0278), the National Natural Science Foundation of China (Grant No. 20471012), a Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China (Grant No. 200322), the Research Fund for the Doctoral Program of Higher Education (Grant No. 20040141004), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, is greatly acknowledged. Supporting Information Available: Detailed experimental conditions in this work, an SEM image, a TG-DSC curve, IR
LiNbO3 Particles with a Tunable Shape spectra, and EDS spectra of the LN intermediate as well as geometries of four kinds of amines ligands. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Jun, Y.; Choi, J.; Cheon, J. Angew. Chem., Int. Ed. 2006, 45, 3414. (2) (a) Zhu, Y.; Bando, Y.; Xue, D.; Golberg, D. J. Am. Chem. Soc. 2003, 125, 16196. (b) Zhu, Y.; Bando, Y.; Xue, D.; Golberg, D. AdV. Mater. 2004, 16, 831. (c) Xu, J.; Xue, D. Acta Mater. 2007, 55, 2397. (d) Yan, C.; Xue, D. J. Phys. Chem. B 2006, 110, 25850. (3) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (4) Kodambaka, S.; Tersoff, J.; Reuter, M. C.; Ross, F. M. Science 2007, 316, 729. (5) Yan, C.; Xue, D. AdV. Mater. 2008, 20, 1055. (6) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (7) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353.
J. Phys. Chem. C, Vol. 112, No. 16, 2008 6351 (8) Guarino, A.; Poberaj, G.; Rezzonico, D.; Deglinnocenti, R.; Gunter, P. Nat. Photonics 2007, 1, 407. (9) (a) He, Y.; Xue, D. J. Phys. Chem. C 2007, 111, 13238. (b) Zhang, X.; Xue, D. J. Phys. Chem. B 2007, 111, 2587. (c) Xue, D.; He, X. Phys. ReV. B 2006, 73, 64113. (10) (a) Liu, M.; Xue, D.; Luo, C. J. Am. Ceram. Soc. 2006, 89, 1551. (b) Liu, M.; Xue, D. Solid State Ionics 2006, 177, 275. (11) Zhao, L.; Steinhart, M.; Yosef, M.; Lee, S. K.; Geppert, T.; Pippel, E.; Scholz, R.; Gosele, U.; Schlecht, S. Chem. Mater. 2005, 17, 3. (12) Zeng, H. C.; Tung, S. K. Chem. Mater. 1996, 8, 2667. (13) Camargo, E. R.; Kakihana, M. Chem. Mater. 2001, 13, 1905. (14) Niederberger, M.; Pinna, N.; Polleux, J.; Antonietti, M. Angew. Chem., Int. Ed. 2004, 43, 2270. (15) Liu, M.; Xue, D. Mater. Lett. 2005, 59, 2908. (16) Luo, C.; Xue, D. Langmuir 2006, 22, 9914. (17) Cheng, Z.; Ozawa, K.; Miyazaki, A.; Kimura, H. Chem. Lett. 2004, 33 1620. (18) Kumar, S.; Nann, T. Small 2006, 2, 316. (19) Deng, Z.; Li, L.; Li, Y. Inorg. Chem. 2003, 42, 2331. (20) Sun, T.; Ying, J. Y. Nature 1997, 389, 704.
